1. Field of the Invention
The present invention relates to an antiskid apparatus for use in a brake for a vehicle. More particularly, the present invention pertains to an antiskid apparatus which is so designed that, when a wheel approaches a locked state, the pressure of a braking liquid is reduced in accordance with the level of the pressure within the brake cylinder.
2. Description of the Related Art
In general, an antiskid apparatus is provided at an intermediate portion of a path for transmitting the pressure of a braking liquid in a master cylinder that is actuated by the operation of brake pedal to the wheel cylinders provided for each wheel. In the case of braking such as to bring the vehicle to a sudden stop, the antiskid apparatus controls the pressure of the braking liquid in such a manner that, as the wheel approaches a locked state, the liquid pressure is alternately raised and lowered in order to prevent side slip of the wheel and to thereby ensure the required steering stability and, at the same time, reduce the distance needed for the vehicle to come to a stop after the application of the brakes.
FIGS. 1 to 3 show in combination one example of conventional antiskid apparatuses of the type described above. The illustrated conventional antiskid apparatus comprises a lock detecting section A for detecting the locked state of a wheel, a liquid pressure regulating section B arranged to lower the pressure of a braking liquid in accordance with the result of the detection effected by the lock detecting section A, and a pressure raising section C for re-raising the lowered pressure of the braking liquid.
When the vehicle is in a normal running state, a ball valve member 2 within a valve chest 1 in the liquid pressure regulating section B is pushed up by means of a pin 3a which is rigidly secured to a piston 3 and which is received through an opening 1a, thereby maintaining the opening 1a in an open state and thus providing communication between a port 4 that communicates with a master cylinder and a port 5 that communicates with a wheel cylinder. When, in this state, the braking liquid is supplied to the port 4 from the master cylinder in response to the operation of the brake pedal, the braking liquid is delivered to the port 5 through an inlet chamber 6 in the pressure raising section C, a communicating passage 7 and the valve chest 1 and is then supplied to the wheel cylinder.
In the lock detecting section A, a flywheel 10 is supported by a rotary shaft 8 which is rotated together with an axle as one unit in such a manner that the flywheel 10 is operatively connected to the rotary shaft 8 through a ball-and-ramp mechanism 9.
At the time of braking such as to bring the vehicle to a sudden stop, as the wheel approaches a locked state, a difference in rotation is produced between the flywheel 10 and the rotary shaft 8, and the flywheel 10 is thus moved leftward as viewed in FIG. 3 to press a lever 11 so that the lever 11 is pivoted clockwise about a pivot member 11a thereby actuating a damping valve 12. The damping valve 12 is provided halfway along a discharge passage 15 which extends from a cylinder chamber 13 in the liquid pressure regulating section B to a return port 14 that communicates with a reservoir of the master cylinder. The damping valve 12 normally blocks the flow path but it is opened by the action of the lever 11 in order to allow the braking liquid contained in the cylinder chamber 13 to flow to the reservoir. When the pressure of the braking liquid within the cylinder chamber 13 lowers as the result of a reduction in the amount of braking liquid, the piston 3 descends, and the ball valve member 2 which has been pushed up by the piston 3 now closes the opening 1a, thereby cutting off the transmission of the braking liquid pressure to the wheel cylinder. The liquid is further discharged from the cylinder chamber 13, and the piston 3 is thus lowered. In consequence, the pressure of the liquid within the wheel cylinder is lowered and the wheel is accordingly unlocked. A pump chamber 17 in the pressure raising section C communicates with the cylinder chamber 13 through a communicating passage 16, and the pressure within the pump chamber 17 is also lowered in response to a lowering in pressure within the cylinder chamber 13, thus causing a piston 18 within the pump chamber 17 to descend so as to come into contact with an eccentric cam 19 which is formed integrally with the rotary shaft 8. As a result, the piston 18 is reciprocated by the action of the cam 19 to cause a pumping action whereby the braking liquid in the reservoir is supplied to the cylinder chamber 13 in the liquid pressure regulating section B via the pump chamber 17. Since, at this time, the damping valve 12 is opened, there is no rise in pressure within the cylinder chamber 13.
On the other hand, when the difference in rotation between the rotary shaft 8 and the flywheel 10 decreases as the result of the fact that the flywheel 10 is decelerated by means of a friction plate 20 and that the rotation of the wheel recovers from the decelerated state after being unlocked, the rotary shaft 8 and the flywheel 10 are coupled together so as to return to the interlocked state by means of the ball-and-ramp mechanism 9. In consequence, the damping valve 12 is closed, and the pressure of the braking liquid within the cylinder chamber 13 is raised by the operation of the pressure raising section C, so that the ball valve member 2 is pushed up by the piston 3, thus allowing resumption of the supply of braking liquid to the wheel cylinder.
In this antiskid apparatus, a flow rate control valve 21 is provided between the cylinder chamber 13 in the liquid pressure regulating section B and the damping valve 12 so that the flow rate of the braking liquid which is returned to the reservoir through the discharge passage 15 is controlled in accordance with the pressure within the cylinder chamber 13 in the liquid pressure regulating section B.
As shown in FIG. 1 in an enlarged view, the flow rate control valve 21 has a valve body 22 which is such that the valve body 22 defines an inlet of the discharge passage 15 below the cylinder chamber 13 in the liquid pressure regulating section B. A retainer 23 having a plurality of bores 23a is provided in an opening formed at the upper end of the valve body 22. A piston 25 and a ball valve member 27 are vertically movably accommodated in the valve body 22, the piston 25 being biased toward the cylinder chamber 13 by the action of a spring 24, and the ball valve member 27 being disposed between the distal end of the piston 25 and the retainer 23 and biased counter to the piston 25 by the action of a spring 26. An orifice 28 is provided in the upper side wall of the valve body 22, while a communicating bore 29 having a relatively large diameter is provided in the lower side wall of the valve body 22, and a seat portion 30 which cooperates with the ball valve member 27 is formed at an intermediate position between the orifice 28 and the communicating bore 29. More specifically, the outer diameter of the ball valve member 27 is so set that a slight clearance is provided betwen the ball valve member 27 and the orifice 28 in order to maintain the orifice 28 in an open state at all times. However, the ball valve member 27 closes the communicating bore 29 when it is pressed against the seat portion 30 as shown by the chain line in FIG. 1.
When the wheel approaches a locked state while the vehicle is running on a road surface which exhibits a relatively high coefficient of friction, the pressure of braking liquid within the cylinder chamber 13 is increased, so that the ball valve member 27 is lowered by the liquid pressure to close the seat portion 30, thus allowing the braking liquid to be discharged relatively slowly through the orifice 28 alone. On the other hand, when the wheel approaches a locked state while the vehicle is running on a road surface that exhibits a relatively low coefficient of friction, such as an icy surface, the pressure of braking liquid within the cylinder chamber 13 is relatively low, so that the ball valve member 27 is pushed up by the piston 25, thus allowing the braking liquid within the cylinder chamber 13 to be discharged rapidly through both the orifice 28 and the communicating bore 29.
However, the flow rate control valve 21 of the type described above suffers from the following problems. Since the ball valve member 27 and the piston 25 are discrete members and these members are moved in contact with each other, there is some risk of the force acting on the ball valve member 27 failing to act directly on the piston 25, which involves a further risk of the response of the ball valve member 27 deteriorating. Further, since the piston 25, the ball valve member 27 and the two springs 24, 26 for biasing them are accommodated in the valve body 22 in a complicated combined state, production of the flow rate control valve 21 involves a difficult and costly manufacturing process. In addition, the arrangement shown in FIG. 1 requires that the orifice 28 is provided in an inclined state, and this involves difficulties in machining.